My Frame Design

[Dandy Dan]

I was looking for some opinions/criticisms on my frame design for my tiny canned ham. I've never done this before, so any advice would be helpful.

I'm not a welder, so I plan on paying to have this fabricated.

It will be a 10 foot box with a 3 1/2 foot tongue. I want a drop floor similar to a Scotty with the drop in the main rail to make for easy access through the door. Although the design shows a drop on the driver's side also, it won't be for a door, its to keep symmetry and to add extra storage.

#9 Dexter axle: 10 deg down; electric brakes; 4'4" bracket dimension; 5' 6" hub face
ST205/75R15 tires
Main frame rails (blue): 3" x 2" x 1/4" C-channel
Cross members (green) 2" x 2" x 1/8” angle
drop floor (red) made from the same angle

Ground clearance, assuming full load for axle, would be 13.5” on the main trailer and 7” on the drop down. The axle is rated at 2200lbs and I am aiming for 1600 lbs max so it should set higher.

If anyone knows a company that can fabricate this in the Greater Chicago Area, please let me know.

Thanks
Dan

PS - the tires in the 3d rendering are not to scale - just pulled the model off the internet

[mdk]

Your tongue has only the strength of the weld at the joint where it joins the straight members of the trailer, and the stiffness of the box above it.

That may be enough, but I would want a way to transfer force between the main frame and the tongue with more than just a narrow weld.

[MtnDon]

First I am not a professional engineer.

However, I believe the 3 x 2 x 1/4" C-channel is not the best choice for frame material. Pound for pound a rectangular steel tube, 3 x 2 inches x some lesser wall thickness will be stronger in load carrying and will resist torsional movement much better. Or another way to look at it a steel tube, 3 x 2 inches could weigh less per foot and be stronger than the C-channel. bdosborn built his Boxcar TTT using 3 x 2 x 14 gauge (0.083") steel tubing for frame rails, 3 x 2 x 11 gauge (0.120") for the tongue and 2 x 2 14 gauge for crossmembers. His trailer weighs in near 3000 pounds, has many miles on it and not suffered any structural failures to my knowledge. His build url is can be found under his member ID. The major difference is you have a planned dropped section of floor and he has straight rails. That is where some practical engineering skills come into play. And a lot depends on the qualifications of the person doing the welding.

The angle stock is subject to even more deformation by loading. Welded in place at each the end the center can twist. Welded in place at one end the free end can twist axially. Square tubing would be a better choice. Again, more strength for the same weight, or it can be built lighter.

I am not qualified to design the drop section and so take everything I say as a suggestion to seek information from those with credentials.

For the tongue each tongue member would be stronger if they extended, uncut, under at least the first two front crossmembers. Using a third center tongue extender tube adds immense strength as well.

Just some thoughts. G/L.

DonM

[aggie79]

Dan,

I know you said you wanted easy access, but if you don't mind stepping over the threshold, the following frame design seems much simpler in design:



The build thread on this project begins here: http://www.tnttt.com/viewtopic.php?p=377215#p377215.

Here's another one by Doug Hodder but with a dropped frame on one side:





His build begins here: http://www.tnttt.com/viewtopic.php?p=572123#p572123.

I look forward to your build!

Take care,
Tom

[Dandy Dan]

Thanks for the input everyone! - back to the drawing board.

[KCStudly]

Full penetration butt welds (by a qualified welder) should be stronger than the tube itself, so, as Hodder did, having the tongue and main rails as one is not taboo. It's just easier and less critical to under sling the tongue.

+1 on the tube vs. channel. Intermediate xmbrs from angle are fine if you plan to secure the floor to them; it will stabilize them.

Looking forward to your build!

[angib]

Full penetration butt welds (by a qualified welder) should be stronger than the tube itself.

That's true for static strength, but definitely untrue for fatigue strength - and it's fatigue that kills most trailer frames, not static strength.

However in this design, with the first joint in the A-frame/main rails behind the first cross-member, I wouldn't be too worried about that welded joint.

Fish-plating (adding doublers) across the side of the welded joint would guarantee it won't fail there.

3 x 2 x 1/4 C-channel isn't a bad choice and it has decent strength for its weight, plus it's easy to paint it really thoroughly. But making joints in C-channel is never as nice as in rectangular tube, which I would prefer.

I think the main rail diagonal struts at either end of the drop floor section are more than is necessary - I would go for a simple horizontal-vertical-horizontal arrangement at the drop. The load on the main rails here is less than at the A-frame, so the strength is not that critical.

[mezmo]

Here are a couple thoughts I had for full-width lowered frames
to allow for lower floors. My thinking being, to use the "C" channel
for the ease of complete rustproofing/painting . The different
longitudinal levels would overlap and have fish-plate reinforcements
on the flat vertical sides, and the meeting of the 'upper' and 'lower'
face 'legs' of the "C" channels would be welded together the full length
of the overlapping sections all around. If the "C" channel has the "C"
'upper and lower legs' facing inward, you could build [and insulate] the
floor to be integral with the top and bottom of the "C " channel frame,
thus saving height. Just weld your outriggers to the flat vertical
outfacing sides of the "C" channel, the outriggers possibly being
connected by an appropriately sized "L" angle perimeter - or not-,
depending on how you'd design the wall and floor connection. The
"A" frame would be integral with the first section of the "C" channel
frame. Plywood would cover the top of the "C" channel frame, providing
the flat floor base.

If cost were no factor, I'd get it galvanized. Otherwise, I'd use something
like the POR-15 rust preventative system, or other similar systems, to make
the frame the least possibly susceptible to rusting over time. This approach
also assumes not running anything through the floor - e.g., run wiring in a
conduit above or below the floor as necessary and so on. The floor only
contains the framing it has, the insulation [foam - any gaps filled with gap
sealing foam], and a flat smooth bottom covering and the plywood floor
decking on top. [Click on pics for a larger view.]




This wouldn't be the cheapest, simplest, or necessarily the lightest way, but it
may give you the most Useful lowered floor height. I personally do not like
step-overs or floor wells, as they are just opportunities for tripping,
especially if you are 'blessed' with larger sized feet !

Cheers,
Norm/mezmo

[ctstaas]

Hey Dandy,
I agree with them. Channel iron is super strong and the 1/4" thickness will be over the top. Good job.
Here's how I see it. You have two rectangular surfaces joined by another rectangular surface on another plane. This upside down bridge is where most of the stress will be happening. I would make the drop rectangle into a truss connecting the three parts together. Square tubing might be perfect for this application but you need to consider connecting the square to channel. FYI, when speaking of channel, the legs are called the flange and the up and down part is the web.
Good joinery is good welding and is a little different when working with channel iron vs square tubing. Then it comes down to the right weld in the right place when it's designed. It's hard to think about everything at the same time, so I try to break things down into smaller tasks when I can.
Hope this helps.
Enjoy, Chris